Conventionally, vehicles employing electric motors for their driving sources are known, such as hybrid vehicles, electric vehicles, and fuel cell vehicles. Each of such vehicles is provided with a power storage device such as a battery storing electric power to be supplied to the electric motor. The battery stores electric power generated upon regenerative braking, or electric power generated by a power generator mounted on the vehicle.
Meanwhile, in some vehicles, batteries mounted thereon are supplied and charged with electric power from power sources external to the vehicles, such as power sources of houses. By connecting an outlet provided in a house to a connector (inlet) provided in such a vehicle via a cable, electric power is supplied from the power source of the house to the battery of the vehicle. In the description below, a vehicle with a battery charged by a power source provided external to the vehicle is also referred to as “plug-in vehicle”.
A standard of plug-in vehicles is established by “Electric Vehicle Conductive Charging System General Requirements” (non-patent document 1) in Japan, whereas it is established by “SAE Electric Vehicle Conductive Charge Coupler” (non-patent document 2) in the United States.
As one example, each of “Electric Vehicle Conductive Charging System General Requirements” and “SAE Electric Vehicle Conductive Charge Coupler” establishes a standard regarding a control pilot. A control pilot has a function of notifying a vehicle that an EVSE (Electric Vehicle Supply Equipment) is in a condition to supply energy (electric power), by sending a square wave signal (hereinafter, also referred to as “pilot signal”) from an oscillator to a control pilot wire. An EVSE is equipment for coupling an external power source and a vehicle to each other. For example, when the plug of the EVSE is connected to the power source external to the vehicle and the connector of the EVSE is connected to a connector provided in the vehicle, a pilot signal is output. By means of a pulse width of the pilot signal, the plug-in vehicle is notified of a capacity of current that can be supplied. When detecting the pilot signal, the plug-in vehicle makes preparations to start charging (closes a relay and the like).    Non-Patent Document 1: “Electric Vehicle Conductive Charging System General Requirements”, Japan Electric Vehicle Association Standards (Japan Electric Vehicle Standards), Mar. 29, 2001    Non-Patent Document 2: “SAE Electric Vehicle Conductive Charge Coupler”, (the United States), SAE Standards, SAE International, November, 2001